Rhodium catalysis enantioselective

Keywords Aldehydes Catalysis Enantioselectivity Hydroformylation Intermediates Rhodium... 

Carbon-carbon bond-forming reactions are one of the most basic, but important, transformations in organic chemistry. In addition to conventional organic reactions, the use of transition metal-catalyzed reactions to construct new carbon-carbon bonds has also been a topic of great interest. Such transformations to create chiral molecules enantioselectively is therefore very valuable. While various carbon-carbon bond-forming asymmetric catalyses have been described in the literature, this chapter focuses mainly on the asymmetric 1,4-addition reactions under copper or rhodium catalysis and on the asymmetric cross-coupling reactions catalyzed by nickel or palladium complexes. 

The potential of QUINAPHOS ligands for asymmetric catalysis was assessed in rhodium-catalyzed enantioselective hydroformylation using styrene as a benchmark substrate (Table 2.1.5.2). The catalysts were prepared in situ from [(acac)Rh(CO)2] and 4 equiv of the diastereomeric mixture or the single diaste-... 

Enantioselective arylation of ketimines has been carried out using rhodium catalysis with chiral sulfur-olefin ligands arylboronic acids are added in up to 99/99% yield/cc. 0... 

Scheme 7.52 Domino carbonyl ylide formation-enantioselective reduction reaction catalysed by chiral phosphoric acid catalysis and rhodium catalysis followed by benzoylation.

The first synthesis of planar meto-cyclophanes was achieved in 2007 through rhodium catalysis as well, with moderate yields but excellent enantioselectivities (Scheme 7.2) [6]. 

Chiral Phenanthrolines in Asymmetric Catalysis. Synthetic modifications on the Phenanthroline core can provide access to highly valuable chiral phenanthroline derivatives with important applications in asymmetric catalysis (Scheme 1). For example, LI has been utilized in copper-catalyzed allylic oxidations, L2 in palladium-catalyzed allylation reactions, and L3-type ligands in rhodium-catalyzed enantioselective hydrosilylation reactions of acetophenone. ... 

At the time the chemistry of main group enolates flourished already for a while, that of late transition metals had a shadowy existence in synthetic organic chemistry. Their stoichiometric preparation and the sluggish reactivity - tungsten enolates, for example, required irradiation to undergo an aldol addition [24a] - did not seem to predestine them to become versatile tools in asymmetric syntheses [27]. The breakthrough however came when palladium and rhodium enolates were discovered as key intermediates in enantioselective catalyses. After aldol reactions of silyl enol ethers or silyl ketene acetals under rhodium catalysis were shown to occur via enolates of the transition metal [8] and after the first steps toward enantioselective variants were attempted [28], palladium catalysis enabled indeed aldol additions with substantial enantioselectivity... 

The rhodium catalysis is particularly useful for the Michael reactions of organosilicon reagents with a y5-unsaturated carbonyl compounds. Excellent enantioselectivities have been achieved with and bod, a chiral diene... 

Hayashi and coworkers have developed the highly efficient enantioselective intramolecular [5+2] cycloaddition of heteroatom (O and NTs) alkyne-VCPs 31 under rhodium catalysis [29]. High enantioselectivities of up to >99% ee were achieved for the corresponding cycloadducts 32 by the use of the chiral phosphoramidite ligand 33 in combination with a catalyst such as [RhCKCaH lJi- The reactions were performed at 30 °C in dichloromethane as the solvent and in the presence of sodium tetrakis(3,5-bis(tiilluoromethyl)phe-nyl)borate (Barp) as source of anionic counterion, which produced a series of chiral heteroatom (O and NTs)-tethered cycloadducts in high yields, as shown in Scheme 20.13. 

Efficient synthesis of chiral amines is very important to the pharmaceutical industry and there are several different approaches using asymmetric hydrogenation that can be employed, some of which have been discussed in previous sections. For example, ot-methyl benzylamine derivatives, common to many pharmaceuticals, can be synthesised in excellent yields and enantioselectivities from the corresponding vinyl acetamide usually employing rhodium catalysis. 

However, dien-ynes with bulky substituents to the alkyne terminus follow a different reaction pathway under rhodium catalysis, they are efficiently converted into spirocyclic compounds. In these rearrangements also, the Rh-TolBEMAP catalyst afforded excellent enantioselectivities (e.e. >90%, Fig. 10.35). 

Table 3.12 surveys current industrial applications of enantioselective homogeneous catalysis in fine chemicals production. Most chiral catalyst in Table 3.12 have chiral phosphine ligands (see Fig. 3.54). The DIP AMP ligand, which is used in the production of L-Dopa, one of the first chiral syntheses, possesses phosphorus chirality, (see also Section 4.5.8.1) A number of commercial processes use the BINAP ligand, which has axial chirality. The PNNP ligand, on the other hand, has its chirality centred on the a-phenethyl groups two atoms removed from the phosphorus atoms, which bind to the rhodium ion. Nevertheless, good enantio.selectivity is obtained with this catalyst in the synthesis of L-phenylalanine. 

Rhodium complexes with chiral dithiolato and dithiother ligands have been studied in rhodium-catalyzed asymmetric hydroformylation. In all instances, enantioselectivities were low.391-393 Catalysis with compounds containing thiolate ligands has been reviewed.394... 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

A crucial achievement significantly stimulated the development of the investigation in the field of homogeneous enantioselective catalysis. The Knowles group established a method for the industrial synthesis of I-DOPA, a drug used for the treatment of Parkinson s disease. The key step of the process is the enantiomeric hydrogenation of a prochiral enamide, and this reaction is efficiently catalyzed by the air-stable rhodium complex [Rh(COD)((PP)-CAMP)2]BF4 (Scheme 1.12). 

---